Generally, an optical code reader comprises on the one hand a light source and an optical illuminating and/or scanning path which leads from the light source towards the optical code, on the other hand a photosensitive device or sensor and an optical receiving device, arranged in an optical receiving path which leads from the optical code to the sensor. Everything is placed in a suitable housing, provided with at least one window, through which preferably both the optical illuminating path and the optical receiving path pass, namely both the light which from the light source goes to illuminate the optical code, and the light which from the illuminated optical code returns towards the sensor.
As is known, light is absorbed by the dark elements and diffused by the light elements of the optical code, and therefore the signal generated by the sensor at a light element takes up a high value, while the signal generated at a dark element takes up a low value. The sensor therefore generates an alternating electrical signal whose waveform is modulated by the sequence of light/dark elements of the optical code. There is an analogous behaviour in case of linear optical codes which may be read by non-visible electromagnetic radiation, for example IR or UV.
The optical receiving device typically comprises an objective comprising one or more lenses, for collecting and forming the image of the optical code onto the sensor. The design of the objective fixes important parameters of the reader, such as the field of view and the focusing distance, wherein under “field of view” the region “optically subtended” by the sensor through the optical receiving device is meant, i.e. the region from which the sensor receives light through the optical receiving device.
The displacement of the objective moreover permits the adjustment—automatic or manual—of the focusing distance. Indeed, moving the objective closer to the sensor, the conjugate point of the sensor on the object plane is moved further away and accordingly an improved reading at long distance between the reader and the optical code is obtained; on the other hand, moving the objective away from sensor, the conjugate point of the sensor is moved towards the reader and therefore an improved reading at small-distance between the reader and the optical code is obtained.
Both in the optical code readers of portable type and in those of fixed type, it is appropriate that the reading of the optical code can correctly occur also for different distances between the reader and the optical code. It is therefore appropriate, besides the capability of adjusting the focusing distance, for the optical receiving device to have an adequate depth of field.
Under “depth of field” the range of reader-optical code distances within which the elements of an optical code may be resolved is meant. The depth of field therefore depends, besides on the characteristics of the optical device, also on the resolution of the optical code.
It is furthermore appropriate for a reader to be capable of reading optical codes of different resolution, wherein the “resolution” is measured in terms of the size of the thinner element of the code, called “module” of the optical code.
The optical receiving device typically comprises, upstream or downstream of the objective, a diaphragm meant as an opaque screen stopping the light, having a light passing aperture so that, of a beam of light rays incident on the diaphragm, the rays which pass through the aperture contribute to the formation of the image onto the sensor, while those intercepted are excluded therefrom.
The known linear optical code readers typically comprise, in the optical receiving path, a diaphragm having only one aperture, typically of a rectangular shape, for example as described in U.S. Pat. No. 4,894,523, the entirety of which is incorporated by reference herein, whose smaller side in use is oriented along the optical code direction and whose greater side in use is along the direction of the elements of the optical code (bars and spaces). With reference to a horizontal orientation of the linear optical code direction, the smaller side of the diaphragm is indicated as the “width” and the greater side is indicated as the “height”. Reference is made hereinafter to such an orientation, for the sake of convenience, but without limiting in any manner the generality of the invention.
The use of a rectangular diaphragm for reading a linear optical code permits both an increase in the resolution of the reader, i.e. the capability of reading optical codes of higher resolution, and an increase in the depth of field with respect to the case of a reader without diaphragm, since a reduction of the aberrations along the scanning direction of the optical code is obtained.
According to the required performance of the optical code scanning device, the size of the diaphragm must be optimised.
Reducing the size of the smaller side of the diaphragm or width, an increase in the depth of field is generally obtained. However, simultaneously, there is also a reduction of the amount of light which reaches the sensor and therefore a reduction of the electrical output signal.
By a further reduction of the size of the smaller side of the diaphragm, the negative effects of the diffraction phenomenon are however increased. The diffraction limits the performance of the optical receiving device with consequent decrease of the depth of field. Because of the diffraction phenomenon, in fact, the plane of higher resolution moves away from the geometric focus, approaching the reader. This phenomenon generally causes a loss of resolution of the optical receiving device and a reduction of the depth of field.
The increase of the size of the greater side of the diaphragm or height leads to an advantageous increase of the amount of light which reaches the sensor, keeping the depth of field unaltered; however, in this case, the sensitivity of the optical receiving device to the “tilt angle” increases, wherein under “tilt angle” the angle comprised between the direction of the bars and spaces of a particular optical code being read and the direction perpendicular to the scanning line is meant. Such increase makes a correct reading of the optical code difficult and sometimes impossible when such tilt angle is greater than a threshold value, variable according to the situation.
In summary, a diaphragm with a narrower aperture permits having a better defined image from close up, where the amount of available light is also greater, but cannot be used in an advantageous manner for long-distance reading of the optical code, and vice versa a diaphragm with a wider aperture permits having a better defined image from a distance, but cannot be used in an advantageous manner for small-distance reading of the optical code, especially in case of high resolution optical codes.
Therefore, there remains a need for method of improving the average reading performance of an optical code reader in terms of depth of field.